Immunohistochemical evidence for the interaction between levator ani and pudendal motor neurons in the coordination of pelvic floor and visceral activity in the squirrel monkey.

OBJECTIVE: The purpose of this study was to characterize the spinal distribution of afferent and efferent pathways that innervate the levator ani (LA) muscle in the female squirrel monkey. STUDY DESIGN: Cholera toxin B (CTB) was injected unilaterally into the LA muscle of 5 monkeys to identify primary sensory neurons in the dorsal root ganglia (DRG) and motor neurons in the spinal cord that contribute fibers to the LA nerve. Fluoro-Gold (FG) was injected into the external anal sphincter of 2 of these animals to label pudendal motor neurons (1 of these animals underwent unilateral LA neurectomy before CTB injection). Spinal cord and DRG were processed for immunofluorescence 3 to 7 days after injections. RESULTS: Retrograde transport of CTB from the LA muscle labeled primary afferent neurons in the ipsilateral DRG, their central projections, and motor neurons in the medial portion of the ipsilateral ventral horn of the spinal cord (L7-S2 segments). Injection of FG into the external anal sphincter labeled cells in Onuf's nucleus, primarily in L7. Importantly, CTB-labeled LA motor neurons were virtually absent in Onuf's nucleus, where all pudendal motor neurons are located. CTB-labeled processes were observed within Onuf's nucleus, adjacent to FG-labeled pudendal motor neurons, and appeared to derive from dendrites of LA motor neurons that project into Onuf's nucleus. CONCLUSION: The LA muscle has a distinct innervation with very little or no contribution from the pudendal nerve. The intriguing labeling of LA neural elements within a nucleus that innervates the external urethral and anal sphincters (involved in pelvic visceral control) may represent a neuroanatomic substrate for physiologic integration of spinal and supraspinal inputs for the coordination of pelvic floor and visceral activity.

Innervation of the levator ani and coccygeus muscles of the female rat.

In humans, the pelvic floor skeletal muscles support the viscera. Damage to innervation of these muscles during parturition may contribute to pelvic organ prolapse and urinary incontinence. Unfortunately, animal models that are suitable for studying parturition-induced pelvic floor neuropathy and its treatment are rare. The present study describes the intrapelvic skeletal muscles (i.e., the iliocaudalis, pubocaudalis, and coccygeus) and their innervation in the rat to assess its usefulness as a model for studies of pelvic floor nerve damage and repair. Dissection of rat intrapelvic skeletal muscles demonstrated a general similarity with human pelvic floor muscles. Innervation of the iliocaudalis and pubocaudalis muscles (which together constitute the levator ani muscles) was provided by a nerve (the "levator ani nerve") that entered the pelvic cavity alongside the pelvic nerve, and then branched and penetrated the ventromedial (i.e., intrapelvic) surface of these muscles. Innervation of the rat coccygeus muscle (the "coccygeal nerve") was derived from two adjacent branches of the L6-S1 trunk that penetrated the muscle on its rostral edge. Acetylcholinesterase staining revealed a single motor endplate zone in each muscle, closely adjacent to the point of nerve penetration. Transection of the levator ani or coccygeal nerves (with a 2-week survival time) reduced muscle mass and myocyte diameter in the iliocaudalis and pubocaudalis or coccygeus muscles, respectively. The pudendal nerve did not innervate the intrapelvic skeletal muscles. We conclude that the intrapelvic skeletal muscles in the rat are similar to those described in our previous studies of humans and that they have a distinct innervation with no contribution from the pudendal nerve.

Innervation of the levator ani muscles in the female squirrel monkey.

OBJECTIVE: The purpose of this study was to characterize the innervation of the levator ani muscles in the female squirrel monkey and to investigate its usefulness as an animal model of pelvic organ prolapse. STUDY DESIGN: Eleven nulliparous female squirrel monkeys with no pelvic organ prolapse were used in this study. Detailed pelvic dissections were conducted (n = 3), and the Koelle stain for acetylcholinesterase was used to identify the motor endplate zone in the levator ani muscles (n = 2). Unilateral levator ani (n = 4) and pudendal (n = 2) neurectomies were performed; changes in levator ani muscle mass and myocyte diameter were examined 14 days after neurectomy. Nerve biopsy specimens from each animal were processed for microscopy. RESULTS: The levator ani nerve originated from the S2 spinal root and entered the pelvic cavity adjacent to the pelvic nerve between the flexor caudalis brevis and iliocaudalis muscles. The levator ani nerve then projected caudally and bifurcated to penetrate the iliocaudalis and pubocaudalis. A single motor endplate zone in each muscle correlated with the point of levator ani nerve penetration. The pudendal nerve originated from the S1-S2 spinal roots to innervate the urethral and anal sphincters, clitoris, and perineum, but not the iliocaudalis or pubocaudalis. Significant atrophy and myocyte shrinkage occurred in the iliocaudalis and pubocaudalis ipsilateral to the levator ani nerve transection (P <.05). Pudendal neurectomy produced no levator ani muscle changes. CONCLUSION: Intrapelvic skeletal muscles in the female squirrel monkey are similar to humans and have distinct innervation with no contribution from the pudendal nerve. The squirrel monkey is likely to be a useful model of pelvic organ prolapse and warrants further study.

Innervation of the female levator ani muscles.

OBJECTIVE: The objective of this study was to characterize the innervation of the human female levator ani muscles. STUDY DESIGN: Detailed dissections of the peripheral innervation of the iliococcygeal, pubococcygeal, puborectal, and coccygeal muscles were performed in 12 fresh-frozen female cadavers (aged, 32-100 years) with the use of transabdominal, gluteal, and perineal approaches. Both the pudendal nerve and the sacral nerve roots that enter the pelvis from the cephalic side were followed from their origin at the sacral foramina to their termination. Pelvic floor innervation was described with reference to fixed bony landmarks, particularly the coccyx, the ischial spine and the inferior pubis. Photographs were taken, and nerve biopsies were performed to confirm the gross findings histologically. Biopsy specimens were stained with Masson's trichrome. RESULTS: In each dissection, a nerve originated from the S3 to S5 foramina (S4 alone, 30%; from S3 and S4, 40%; from S4 and S5, 30%), crossed the superior surface of the coccygeal muscle (3.0 +/- 1.4 cm medial to the ischial spine [range, 1.0-4.2 cm]), traveled on the superior surface of the iliococcygeal muscle innervating it at its approximate midpoint, and continued on to innervate both the pubococcygeal and puborectal muscles at their approximate midpoint. The pudendal nerve originated from the S2 to S4 foramina, exited the pelvis through the greater sciatic foramen, traversed Alcock's canal, and branched to innervate the external anal sphincter, the external urethral sphincter, the perineal musculature, the clitoris, and the skin. Despite specific attempts to locate pudendal branches to the levator ani, none could be demonstrated. Nerve biopsy specimens that were obtained at gross dissection were confirmed histologically. CONCLUSION: Gross dissections suggest that the female levator ani muscle is not innervated by the pudendal nerve but rather by innervation that originates the sacral nerve roots (S3-S5) that travels on the superior surface of the pelvic floor (levator ani nerve). Because definitive studies (eg, nerve transection or neurotracer studies) cannot be performed in humans, further studies that will use appropriate animal models are necessary to confirm and extend our findings.